This invention relates to removable pressure sensitive adhesives based on aqueous emulsions. The removable pressure sensitive adhesives of this invention have enhanced plasticizer resistance properties which are particularly useful for pressure sensitive adhesive applications on plasticizer containing vinyl films.
Removable pressure sensitive adhesives are used on a variety of products including labels, tapes, films, and the like, to enable the product to adhere to a substrate and then later to be removed from the substrate without difficulty and without leaving a stain or residue. To be suitable for such applications, the pressure sensitive adhesive must have good adhesion with low initial peel strength and must not exhibit a significant increase in adhesion over time. Optimally, the pressure sensitive adhesive should exhibit such physical properties on a variety of substrates. Typical commercially available aqueous emulsion removable pressure sensitive adhesives have anchorage and removability weakness when used with plasticizer containing facestocks such as vinyl films.
Plasticizers usually are moderately high molecular weight organic liquids, or occasionally low melting solids, which are incorporated in a material to increase its workability, flexibility, or distensibility. For example, polyvinyl chloride (PVC) or, as it is more commonly referred to, “vinyl”, in its unmodified form, is typically rigid. When compounded with a plasticizer, it becomes more flexible and can be used for broader applications. Plasticized vinyl typically contains 15-50% by weight of either a monomeric or polymeric plasticizer, with the monomeric plasticizers being more commonly employed. The monomeric plasticizers are of relatively low molecular weight and tend to migrate to the surface of the plasticized vinyl. When a typical pressure sensitive adhesive is applied to a plasticized vinyl film and allowed to remain in contact with it for an extended period of time, plasticizer from the vinyl tends to migrate into the pressure sensitive adhesive, softening it and causing a decrease in adhesion. Pressure sensitive adhesives that have high initial peel strength are more resistant to plasticizer, however, the resulting decreased removability is undesirable.
Attempts have been made to solve the plasticizer migration problem by interposing an impenetrable barrier between the vinyl and the pressure sensitive adhesive (“PSA”); see, e.g., U.S. Pat. Nos. 4,045,600 and 4,605,592. Others have developed pressure sensitive adhesives that are said to be less susceptible to weakening by plasticizer, see U.S. Pat. No. 4,985,488. Another attempt to prevent migration of plasticizer from a plasticized vinyl substrate into a confronting pressure sensitive adhesive has been to incorporate plasticizer into the pressure sensitive adhesive, thereby minimizing the plasticizer gradient between the contacting layers; see, e.g., published European Pat. App. No. EP 150,978 A, and PCT publication no. WO 00/36043.
Despite the limited success achieved by products of the type discussed above, there has remained a strong commercial desire for pressure sensitive adhesives that not only maintain strong anchorage to a variety of vinyl films, but also maintain good removability.